What Really Happened in Steve Rogers’ Lost Timeline?

Brazilian MCU fan here.

I was rewatching Avengers: Endgame and realized something that Marvel itself never properly explained.

According to Endgame’s own time travel rules, Steve Rogers should not have been sitting on that bench at the end of the movie.

Bruce Banner explains that traveling to the past does not change your future. Instead, it creates a new timeline.

That’s exactly why Loki’s escape created a branch timeline.

It’s why the 2014 version of Thanos created another branch timeline.

It’s the entire foundation of Endgame’s time travel rules.

So here’s my question:
How did Steve Rogers return to Peggy Carter, spend an entire lifetime in the past, and then somehow appear as an old man in the original MCU timeline?

Even Marvel’s writers and directors have given different explanations over the years.
Some say Steve lived in an alternate timeline and later returned.
Others imply he was somehow always part of the same timeline.
The truth is that the MCU never clearly explained it.
And I think that’s where one of the most interesting stories in Marvel history begins.
Because if Steve really created a new timeline, then what happened inside that reality?
Think about it.
Steve returns to the late 1940s with complete knowledge of the future.
He knows:
HYDRA has infiltrated SHIELD.
Bucky Barnes is alive and being used as the Winter Soldier.
Howard and Maria Stark will be murdered.
Loki will invade Earth.
Ultron will be created.
The Infinity Stones exist.
Thanos is coming.
Now ask yourself:

Would Steve Rogers really stay silent?
This is the same man who spent his entire life fighting against injustice.
The same man who destroyed HYDRA.
The same man who spent years searching for Bucky.
The same man who never accepted standing by while innocent people suffered.
So are we really supposed to believe that Steve spent decades watching history repeat itself?
I don’t buy it.
I think Steve would interfere.
I think he would save Bucky.
I think he would expose HYDRA decades earlier.
I think he would prevent the murders of Howard and Maria Stark.
And I think he would change history forever.
That idea led me to a movie concept:

Captain America: The Lost Timeline
A World That Should Never Exist
A reality where Steve Rogers refuses to stay silent.
A reality where Bucky is rescued years before becoming the Winter Soldier.
A reality where HYDRA is exposed before it can take control from within.
A reality where countless tragedies never happen.
A reality that slowly becomes completely different from the MCU we know.
More advanced.
More stable.
More united.
But also more dangerous.
Because every life saved changes history.
Every event prevented creates new consequences.
Some heroes never become heroes.
Some villains never become villains.
Entire MCU events may never happen.
The Avengers themselves might not even form in the same way.
And eventually, this impossible reality becomes powerful enough to attract the attention of the multiverse itself.
For me, this isn’t just a movie idea.
It’s the missing chapter of Steve Rogers’ story.
The story Marvel may have accidentally created, but never told.
And one more thing:
If Steve Rogers really spent an entire lifetime in an alternate timeline with Peggy Carter, how did he end up sitting on that bench in the original MCU timeline at the end of Endgame?
Marvel never gave us a clear answer.
Maybe the answer is hidden inside the Lost Timeline itself.
Would you watch Captain America: The Lost Timeline?